If you are trying to add this Iframe on a SSL-encrypted website (https://), it won't work any more since Firefox 23 because Mozilla has decided to blocked all unencrypted content on encrypted websites (for example http-iframes on https-websites). You can change this behaviour in your own Firefox installation by typing about:config in the address bar and setting security.mixed_content.block_active_content to false. But that won't help you for all other FF23-visitors on your website.